The 30-year evolution of airway pressure release ventilation (APRV)
BACKGROUND Prone positioning has been shown to improve oxygenation in 60-70% of patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). Another way to improve matching of ventilation to perfusion is the use of partial ventilatory support. Preserving spontaneous breathing during mechanical ventilation has been shown to improve oxygenation in comparison with controlled mechanical ventilation. However, no randomized studies are available exploring the effects of preserved spontaneous breathing on gas exchange in combination with prone positioning. Our aim was to determine whether the response of oxygenation to the prone position differs between pressure-controlled synchronized intermittent mandatory ventilation with pressure support (SIMV-PC/PS) and airway pressure release ventilation with unsupported spontaneous breathing (APRV). METHODS We undertook a prospective randomized intervention study in a medical-surgical adult intensive care unit of a university hospital. Of 45, 33 ALI patients (acute lung injury) within 72 h after initiation of mechanical ventilation, and in whom the prone position was applied according to a predefined strategy, were included in the study. After initial stabilization the patients were randomized to receive either SIMV-PC/PS or APRV with predefined general ventilatory goals (PEEP, tidal volume, inspiratory pressure and PaCO2-level). The protocol for prone positioning was the same for both treatment arms. Prone positioning was triggered by finding a PaO2/FiO2-ratio below 200 mmHg evaluated twice per day. The duration of each prone episode was 6 h. RESULTS The first two episodes of prone positioning were analyzed. Gas exchange was measured before and at the end of prone positioning. Of the 45 patients enrolled, 33 were turned prone once and 28 twice. No significant differences were detected in baseline characteristics. Changes in oxygenation were analyzed in response to the first and second prone episodes 5 h and 24 h after randomization and initiation of SIMV-PC/PS or APRV respectively. Before the first prone episode the PaO2/FiO2-ratio was significantly better (P = 0.02) in the APRV-group (median; interquartile range) (162; 108-192 mmHg) than in the SIMV-PC/PS-group (123; 78-154 mmHg). The response in oxygenation to the first prone episode was similar in both groups: PaO2/FiO2-ratio increased 39.5; 17.75-77.5 mmHg in the SIMV-PC/PS-group and 75.0; 9.0-125.0 mmHg in the APRV-group (P = 0.49). Before the second prone episode, the PaO2/FiO2-ratio was comparable (SIMV-PC/PS 130.5; 61.0-161.0 mmHg vs. APRV 134; 98.3-175.0 mmHg). Improvement in oxygenation was significantly (P = 0.02) greater in the APRV group (82; 37.0-141.0 mmHg) than in the SIMV-PC/PS group (50; 24.0-68.8 mmHg) during the second prone episode. General ventilatory and hemodynamic variables and use of sedatives were similar in both groups during the study. CONCLUSIONS APRV during prone positioning is feasible in the treatment of ALI patients. APRV after 24 h appears to enhance improvement in oxygenation in response to prone positioning.